I still don’t understand the using HTA to adjust cockpit thing. But I think at this point it’s probably not productive to keep discussing it. I also use front center/rear center as my primary independent variables, along with HTA. Stem length is a dependent variable.
Are you doing the same thing? It sounds like maybe yes?
-Walt
For the hardtails I build I generally default to 76 STA / 64 HTA and go from there. I don’t find there to be much difference in a 64-66 HTA when building bikes with front centers longer than 800mm, especially when your front center isn’t dictated by HTA as I showed in the drawings. If a rider wants a more chill ride, I just dial the front center back and steepen the HTA by .5 / 1 / 1.5 / 2 degrees. Of course that only goes so far, as in the drawings I posted you can see it only changes by 11mm over 1 degree. Obviously you can over do it as well. Let’s call HTA the fine tuner on my violin, small adjustments are better. A slight adjustment can be beneficial for lengthening cockpit without changing ride characteristics, and it is only one of many parameters I use to adjust cockpit.
That hit the nail on the head haha. It takes a very open-minded person to convince that they need a steeper HTA and longer chainstays than everyone else! We have been lucky that our customers trust us.
Good question, I was hoping someone would ask 
. Rear-front-ratio is just one of many parameters we look at. Without getting too much into our secret sauce, ill just say bar heights, BB drop, wheelsize, compliance, and DFM also impact our decisions for the front-rear centers.
I can share an interesting story. @Neuhaus_Metalworks and do a lot of testing with shorter riders (<5’4). We have built 4 small prototypes, all with slightly different geometry, and have tested them with multiple riders.
Going into it, my theory was that the short bikes would need shorter chainstays to have a “normal” weight bias. We built two prototypes with 405mm chainstays. Immediately, the number one comment: the rear end felt too harsh.
In retrospect, it makes sense. Shorter chainstays make the rear end stiffer. Also, more weight on the rear wheel means more force gets transmitted through the frame. These riders are typically <130lbs, so they are more susceptible to stiff frames. Making the chainstays shorter was a step in the wrong direction for those sizes.
Moral of the story: Always test your assumptions
Josie testing one of our early prototypes!:
Several years ago I made a hardtail for a 5’1" friend of mine. The goal was to make her able to bunny hop by making a frame with 405mm chainstays. The frame ended up with a 1.64 FC/RC ratio. And yet she was still unable to get the front wheel off the ground 
I think the FC/RC ratio takes a back seat to other priorities at the extreme ends of frame sizing. As @Daniel_Y noted, short stays make the ride harsher.
For the XL and above riders, they’ve likely never ridden a bike with a FC/RC ratio we’d consider normal in the M and L sizes. And because they’re laying out a lot of $ for a new frame, they aren’t likely to take a big risk and ride something they’re not used to. They’ve adapted to riding frames that are more rear wheel biased for better or worse.
I feel like my job as a builder is to make suggestions in the design that I think would be an improvement for the customer. But if the customer is determined to stick with a 30mm stem and 430mm chainstays, then I’ll acquiesce. After all, the bike is for the customer, not me.
Yep, so long as it’s not dangerous, the customer is always right.
That said, I’ve fired a decent number of customers over the years. If it’s really going to suck I don’t want my name on it.
-Walt
I was curious what AI thinks about chainstay length, so I asked it:
On a hardtail mountain bike, the chainstay length can have a significant impact on the bike’s agility and stability. A shorter chainstay length will generally result in a bike that is more agile and easier to maneuver, as it allows for a quicker response to rider input. This can be especially useful for technical and tight trails, where a quick and responsive bike can be an advantage.
On the other hand, a longer chainstay length will generally result in a bike that is more stable and better suited for high-speed descents. This is because the longer chainstays help to distribute the rider’s weight more evenly between the front and rear of the bike, which can improve traction and stability at high speeds.
It’s worth noting that there is no “ideal” chainstay length that is right for everyone, as the ideal length will depend on the rider’s preferences, riding style, and the types of trails they are riding. Some riders may prefer a shorter chainstay length for its agility, while others may prefer a longer chainstay length for its stability. Ultimately, it’s up to the individual rider to decide which chainstay length is best for them.
Sad to admit, the AI is smarter than me
Kris Henry made a pretty substantial write up on his process recently, thought you all might enjoy.
That is an excellent article. Much more patient than me I think, such fine adjustments over many iterations.
I compartmentalize (somewhat) my fit and handling dimensions.
For fit, I focus on saddle to bar center, saddle height and bar drop. I pay attention to setback while designing the bike, but it is so adjustable with seat posts and saddles I wouldn’t consider it an entirely driving dimension. So, I get the rider’s butt, feet and hands where I want and then smoosh the geometry around to maintain fit and make the bike handle well.
For handling geometry the big things I am looking at are front center, trail vs stem length, BB height (not drop) and the overall wheelbase/rear center. I usually start with front center, and I hate to say it so unscientifically, but I think I have just gotten a feeling for what numbers work well for different rider sizes and riding styles. I went coocoo on PVD geometry a few years back and found that those bikes are super boring to ride so I’ve really reeled it in. It really helped me get a feel for how FC affects handling though. Of course a lot of customers are looking for that geo these days so I always try to balance what they are telling me/asking for with what I think will ride well. Anyway, I do agree with the whole stem length/trail lever vs lever philosophy. I design almost all of my MTB’s around 35mm stems unless the rider is really tall/army. I tweak that ratio with HTA and often times rake if I am building the fork, while maintaining a nice fit. I find with gravel bikes I usually set the BB height first based on rider preferences and then build the geo around that, but with MTB’s I am usually tweaking it a lot throughout the process.
Geo is so fun because you can’t change one thing without changing the whole thing. It sure is fun to get in there and tweak things around, look at it from all the different angles. Fun stuff.
@adamsklar I’ve definitely swallowed the PVD pill, mainly because his approach is so comprehensive. I’m a first timer so I my understanding of geo is basically nill, and a singular vision in a sea of opinions is very compelling. What was it about the bikes you produced using his method that you disliked?
@wizman I am all the way behind the PVD design approach/methodology. No one has put it all out there so comprehensively as that guy. The process is logical and about as objective as you can get with this stuff. My approach is really similar to Peter’s and that is because I learned it from him. That said, we have different goals and metrics for what makes a successful bike design. As an engineer myself I respect the desire to optimize around something measurable, in Peter’s case that seems to be speed. The bikes I built way out on the PVD end of the spectrum were definitely fast but I’ve found myself coming to the philosophy that “fast is fun but fun is funner”. Those bikes are great for riders who want to point bikes down steep trails and keep their wheels glued to the ground. To me they were boring to ride because I like to ride my bike for fun.
I’ve been thinking of riding styles on a spectrum from “the feeling of riding a bike” on the left and “full PVD” on the right. Personally I fall about 80% to the right on that scale. Placing whoever you are building a bike for on that spectrum is a fun challenge as a designer and once you do that, designing the bike is easy. A lot of people just want that familiar feeling of an “old fashioned” bike and I don’t blame them. Riding a bike at all is sentimental in a lot of ways, and those folks can be super happy on a bike we might consider outdated. If you give them an 800FC road bike with a 31mm stem they are going to hate it. Sure they could be “performing better”, but that is super not the point of riding a bike for 99.9999% of people who ride bikes. So TLDR; I love sneaking in components of long FC bike design, it makes sense in some cases but people want to ride something familiar to have a good time. Or something like that…
Yeah, I often use surfboard shaping as an analogy. Outside of some fringe cases (big wave stuff where you need to make it down the face of the wave fast, racing standup boards) surfers don’t care about whether their board is fast, they want it to be capable at doing the stuff they prefer to do (which obviously varies a lot) and that it gives them the feeling they’re after.
A lot of my favorite days on the bike in my life weren’t winning races or going as fast as I possibly could, but achieving that perfect flow speed (most fun with a few friends the same speed!) where you’re never really pushing crazy hard or worried about crashing but you just feel like you’re floating/flying along. I don’t know if that is 80 or 90 percent of full race speed but I’d guess that’s in the ballpark for me personally.
I tend to agree that the really super long PVD style bikes (I do own one that is somewhat in that category) can be, essentially, numb feeling on the trail. I don’t want my trail rides to feel like I’m on pavement (nor do I want to ride full rigid on 40c tires at 60 psi, of course) even if a 170mm travel long/low/slack bike is objectively faster on some of those trails.
There are always exceptions, though. We do have a lot of high elevation moto trails around here, and for that (chunder and whoops!) going as long and slack as you can is great, though, because anything less is nigh-unrideable.
-Walt
Agreed, and great points.
I don’t believe there is a single optimal solution because everyone has their own cost function they are trying to optimize: climbing, descending comfort, weight, speed, stability, nimbleness, etc… Many of those parameters are in direct conflict with each other.
Riding is also a moving target that changes through time. Your riding style, fitness, equipment, technology, fashion, and even the trails are all in a constant state of flux. That is why I think it’s always great to question old ideas and try new things.
That’s why mountain biking is awesome and building mountain bikes still keeps my interest after all these years.
A road bike from 1970 (or probably 1930, but I’ve never ridden one) rides basically the same as a road bike from today (and fits 30-40c tires, too, so it’s all-road AF!) because there’s just not that much that a road bike has to do. 73/73 and call it done. Boring.
It’s too bad for me that roadies think nothing of dropping $5k on a wheelset and my mountain bike customers were always asking if they could get just a $15 left brake lever to go with their still-ok caliper from 2004. I should have been thinking about shorelines and pin-striping, I guess. 
-Walt
I don’t agree that road bikes are done and boring. Maybe road race bikes.
I think that the popularity of touring/gravel/all-road has been interesting to follow. There are basically two paths: the road bike with wider tires and optionally low trail path (UK rough stuff bikes in the 70s, Bridgestone XO-1 in the 90s, Rawland and Elephant in the noughts, then everyone starting a few years ago) or the mountain bike geo with drop bars (evil, new Marin, etc). It’s still evolving.
I personally love riding my flexy low trail steep head tube angle bikes on trails. They make green trails fun again and are a lot faster on pavement. It’s a fun low risk challenge to clean easier but muddy trails on fat slick tires. I also love riding long FC slacker HT bikes on mixed surfaces, but the one that I have is a lot slower from its stiffer tubing, wide bars, and upright riding position. It’s more capable on trails though and works for riding with my kids (3 and 5) on a Mac-Ride seat. Since I like to mix gravel into almost every ride (and seat streets are awful) I ride some flavor of mixed surface bike on pretty much every ride.
Yes, flexy low trail steep head angle bikes on easy trails/gravel has been a thing since the 1800s. It was great then, still great now. But not very exciting from a design perspective.
-Walt
I also have a ton of fun bringing over-leveraged ‘road bikes’ into green / blue trail territory. I’ve even ridden my Bullitt Cargo bike on MTB trails to do trail building or setting up events.
I agree with your statement that longer FC drop bar bikes with a short stem are just a blast to ride on pavement, and living in an area with a ton of climbing means I can take the pavement up and blast down on the gravel / mtb trails.
Just in terms of angles, I’ve been trying out 68HTA and 75STA on these kind of bikes. Coupled with a 35-42mm stem.
I’ve been riding a flat bar bike with almost that geo (69/74) for the last month or so. It also has a dropper with zero offset, so the STA is a lot steeper than any of my other pavement oriented bikes. I think the STA is a little too steep, my upper body tires out on it faster than on my 73 sta bikes.
I really get the long FC slack/steep thing for mountain bikes where you only go up and down. I don’t really like it on pavement.
I really enjoy steeper STA for climbing. I have a bad knee injury (was hit by a bus on my bike) and find riding slack seat tube angles aggravates it, especially with 170-175mm cranks. Almost all of my riding has been on MTB bars, so I only really ride in the drops when I want to go faster on downhills. My body is probably very use to steep STA since all of the bikes I’ve put in real kilometers on have been 75-77STA in the last four years. Prior to the injury I did ride a lot of 73-74°STA bikes, but personally I really enjoy the feeling of being ‘on top’ of the bottom bracket, along with it not aggravating my body.
I find that a steeper STA (let’s say 75°+) gives me the feeling of pushing down with my feet, where around 73° makes me feel like I’m pushing forward. If I spend a day riding a friend’s bike with a slack STA and long cranks I can’t walk for a few days!
